Midian
Moses's father-in-law was called a priest of Midian in the Pentateuch, but in Judges he is called a Kenite. The Midianites in Judges 6 are raiders allied with Amalekites and other "eastern peoples". They live in tents, invade the land, and camp there, raiding crops (Judges 6:3-5). These raiders wear gold earrings and are called Ishmaelites (8:23). The Midianite leaders are named Oreb and Zeeb (7:25). Their two kings are named Zebah and Zalmunnah (8:5,12). They are also known as nomads (8:11). Their warriors fought with swords (8:10). There were originally 135,000 in their "armies of the eastern peoples", but 120,000 were killed (8:10) and the remaining 15,000 were routed by Gideon's forces (10:12). They spoke a language which Gideon could understand (7:13-14).
The angel of Yahweh
Yahweh had promised to send his angel ahead of Israel "to guard you along the way and to bring you to the place I have prepared" (Exodus 23:20). The Israelites were to "[p]ay attention to him and listen" in order that the angel would drive out the Canaanites. The angel came to the Israelites at Bokim in the first part of Judges, and berated them for not obeying and for letting the Canaanites remain and for worshiping other gods. The Israelites mourned and wept, but didn't really do anything. Later on, the angel appeared to Gideon, and told him to be courageous because God was with him, and to fight and defeat the Midianites. Yahweh, I guess through the angel, later told him to tear down Baal and Asherah's altar in his hometown and to erect a proper altar to Yahweh. And he did so.
Anxiety
God reassured Gideon's anxiety and gave him courage in several ways, including words from the angel, the test of tearing down Baal's altar and not being punished for it by the townspeople after his dad defended him, the two episodes with the fleece, and the Midianite's prophetic dream. God lessened Gideon's confidence, however, by making him send most his army away -- this was so that his confidence would not be in the size of his army, but in the Lord. And by the end, much of northern Israel was joining his army anyway, although Ephraim was pissed at him for not inviting them sooner, and the men of Succoth and Peniel didn't want to give him provisions because he hadn't completely defeated the Midianites yet. In sum, God provided words, signs, and small tests to increase his confidence in God, and yet through those tests, he temporarily diminished his confidence in people.
Mountains, valleys, and rivers
In Gideon's story, like in many other episodes in Judges, Israel's enemies camped en masse in a valley or on a plain, while the Israelites assembled on a mountain and then came down to fight. The Israelites chased their enemies (or the enemies chased them) to a river, and then more Israelites guarded the fords of the river and killed their enemies as they tried to cross. This may relate to how the Israelites took possession of much of the hill country but the local non-Israelites retained control of the plains.
The enemy's animals
The non-Israelites often had military equipment and swift animals, such as camels (in the case of Midian) and horses and chariots (in the case of other nations), but these didn't determine the outcome of the battle when God was on the side of Israel.
Confusion in the enemy's camp
In the story of Gideon, and later on in 1 Samuel with the story of Jonathan, the enemies of Israel are taken by surprise and are so confused that they start fighting each other and running away, even before a very small number of Israelites. This is also meant to show that God put them in a panic and the battle was not determined by the number of Israelite fighters.
